Photoionization of hot radicals: C2H5, n-C3H7, and i-C3H7

The combination of ion-imaging and vacuum-ultraviolet (vuv) single-photon ionization is used to study the internal energy dependence of the relative photoionization yields of the C(2)H(5),n-C(3)H(7), and i-C(3)H(7) radicals following the 266 nm photodissociation of the corresponding alkyl iodides. The comparison of the ion images obtained by vuv photoionization of the radical with those obtained by two-photon-resonant, three-photon ionization of the complementary I (2)P(32) and I*(2)P(12) atoms allows the extraction of the internal energy dependence of the cross sections. Factors influencing the appearance of the ion images in the different detection channels are discussed, including the secondary fragmentation of the neutral radicals, Franck-Condon factors for the photoionization process, and the unimolecular fragmentation of the parent photoions.     

Isomeric structures and visible electronic spectrum of the C7H3 radicals

The 2-(buta-1,3-diynyl)cycloprop-2-yl-1-ylidene radical, a new three-membered ring chain with Cs symmetry, has been detected by electronic spectroscopy in the gas phase. The experimental investigation used a mass selective resonant two color two photon ionization technique coupled to a supersonic plasma source. Structures and relative stability energies of eight isomers of the C7H3 radical have been calculated. Based on the rotational analysis and the theoretical calculations, the observed spectrum is assigned as an 2A" <-- X2A' electronic transition of this exotic chemical species. This result shows that such a plasma source is a powerful tool to investigate intermediates involved in hydrocarbon chemistry as in flames.     

Structure, stability, and spectra of C9H3, C11H3, and C13H3 radicals

Density functional theory has been used to investigate the geometries, vibrational frequencies, rotational constants, and dipole moments of the C(9)H(3), C(11)H(3), and C(13)H(3) radicals. Vertical electronic transition energies of C(9)H(3), C(11)H(3), and C(13)H(3) are calculated by the time-dependent density functional theory. Present results show that the most stable arrangements of C(9)H(3), C(11)H(3), and C(13)H(3) are H(2)C(9)H, H(2)C(11)H, and H(2)C(13)H with a C(2v) symmetry, respectively. Such lowest-energy isomers have an obvious single and triple bond alternation carbon chain. Their isomers HC(4)(HC)C(4)H, HC(4)[C(C(2)H)]C(4)H, and C(C(4)H)(3) are predicted to have vibrational frequencies and vertical excitation energies in good agreement with experimental observations. HC(4)(HC)C(4)H, HC(4)[C(C(2)H)]C(4)H, and C(C(4)H)(3) have similar trigonal structure, which gives rise to the remarkably similar spectroscopic features as obtained experimentally. On the basis of present calculations, the isomers HC(4)(HC)C(4)H, HC(4)[C(C(2)H)]C(4)H, and C(C(4)H)(3) of C(9)H(3), C(11)H(3), and C(13)H(3) radicals are most likely the carriers of the observed spectra.     

Association rate constants for reactions between resonance-stabilized radicals: C3H3 + C3H3, C3H3 + C3H5, and C3H5 + C3H5

Reactions between resonance-stabilized radicals play an important role in combustion chemistry. The theoretical prediction of rate coefficients and product distributions for such reactions is complicated by the fact that the initial complex-formation steps and some dissociation steps are barrierless. In this paper direct variable reaction coordinate transition state theory (VRC-TST) is used to predict accurately the association rate constants for the self and cross reactions of propargyl and allyl radicals. For each reaction, a set of multifaceted dividing surfaces is used to account for the multiple possible addition channels. Because of their resonant nature the geometric relaxation of the radicals is important. Here, the effect of this relaxation is explicitly calculated with the UB3LYP/cc-pvdz method for each mutual orientation encountered in the configurational integrals over the transition state dividing surfaces. The final energies are obtained from CASPT2/cc-pvdz calculations with all pi-orbitals in the active space. Evaluations along the minimum energy path suggest that basis set corrections are negligible. The VRC-TST approach was also used to calculate the association rate constant and the corresponding number of states for the C(6)H(5) + H --> C(6)H(6) exit channel of the C(3)H(3) + C(3)H(3) reaction, which is also barrierless. For this reaction, the interaction energies were evaluated with the CASPT2(2e,2o)/cc-pvdz method and a 1-D correction is included on the basis of CAS+1+2+QC/aug-cc-pvtz calculations for the CH(3) + H reference system. For the C(3)H(3) + C(3)H(3) reaction, the VRC-TST results for the energy and angular momentum resolved numbers of states in the entrance channels and in the C(6)H(5) + H exit channel are incorporated in a master equation simulation to determine the temperature and pressure dependence of the phenomenological rate coefficients. The rate constants for the C(3)H(3) + C(3)H(3) and C(3)H(5) + C(3)H(5) self-reactions compare favorably with the available experimental data. To our knowledge there are no experimental rate data for the C(3)H(3) + C(3)H(5) reaction.     

Rotational spectra of the carbon-chain radicals HC5O, HC6O, and HC7O

Three new free carbon-chain radicals, HC5O, HC6O, and HC7O, and their deuterated isotopic species have been observed by Fourier transform microwave spectroscopy of a supersonic molecular beam. In contrast to the shorter HCnO radicals, these all have linear heavy-atom backbones and 2Pi electronic ground states. Like the isovalent HCnS radicals, the ground states of the HCnO radicals alternate with odd and even numbers of carbon atoms: those of HC5O and HC7O are 2Pi1/2 and that of HC6O is 2Pi3/2. From frequency measurements between 6 and 26 GHz, the rotational constant B, the centrifugal distortion constant D, and the lambda-type doubling and magnetic hyperfine constants have been determined to high precision for each chain. Predicted properties from coupled-cluster calculations are also reported for chains up to HC9O. The production of HCnO radicals for n even was highly favored when O2 was used as the source of oxygen, but those with n odd were best produced with CO.     

Rate constants for tsomerization of C6H5C(SC6H5)2 ĆH2 radicals to C6H5Ć(SC6H5)CH2SC6H5 radicals based on EPR data

Conclusions The parameters of the Arrhenius equation for the isomerization of PhC(SPh)₂H₂ radicals to Ph(SPh)CH₂. SPh radicals are in agreement with an intramolecular character of the rearrangement, with a 1,2-migration of the thiyl group. These parameters were calculated on the basis of the data that were obtained by the EPR method in the range 10–70°.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2596–2597, November, 1982.     

[C6H6, C3H7 +]and [C6H7 +, C3H6] ion-neutral complexes intermediate in the reactions of protonated propylbenzenes

From the mass-analysed ion kinetic energy spectra of labelled ions, kinetic energy releases and thermodynamic data, it is proved that protonated n-propylbenzene (1) isomerizes into protonated isopropyl benzene (2). It is also shown that the dissociation of the less energetic metastable ions of (2), leading to [iso-C₃H₇]⁺ and [C₆H₇]⁺ product ions, is preceded by H exchange. This H exchange involves two interconverting ion-neutral complexes [C₆H₆, iso-C₃H₇⁺] (2π) and [C₆H₇⁺, C₃H₆] (2α).     

Absorption spectrum of Yb3+(4f13) IN[(C6H5)3PH]3 YbCl6

Optical absorption spectra taken at 300 and 77 K are reported for six-fold octahedrally coordinated Yb³⁺(4f¹³) in [(C₆H₅)₃PH]₃ YbCl₆. In addition to vibronic spectra we observe electronic transitions which suggest that inversion symmetry is lifted by a small distortion similar to that reported for the Nd³⁺(4f³) spectrum of the corresponding salt. The vibronic as well as the electronic transition have been analyzed on the basis of six-fold octahedral symmetry. The analysis appears reasonable and consistent in comparison with other rare-earth ions that have been studied in similar cubic and octahedral environments.     

Accurate ab initio predictions of ionization energies of hydrocarbon radicals: CH2, CH3, C2H, C2H3, C2H5, C3H3, and C3H5

The ionization energies for methylene (CH2), methyl (CH3), ethynyl (C2H), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), and allyl (C3H5) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation [CCSD(T)]. When it is appropriate, the zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect correction, the diagonal Born-Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy (IE) values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron (PFI-PE) studies indicates that the CCSD(T)/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within +/-10 meV. The benchmarking of the CCSD(T)/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck-Condon factors for photoionization transitions near their ionization thresholds.     

Tungsten monocarbide, WC: pure rotational spectrum and 13C hyperfine interaction

The J = 1→2 pure rotational transitions in the X(3)Δ(1)(ν = 0) state of (186)W(12)C and (184)W(12)C were recorded using a pump/probe microwave optical double resonance (PPMODR) technique and analyzed to give fine structure parameters. The field-free [17.6]2←X(3)Δ(1) (1, 0) bands of the W(13)C isotopologues were recorded using laser induced fluorescence and analyzed to produce the (13)C(I = 1/2) magnetic hyperfine parameter. Bonding in the [17.6]2(ν = 1) and X(3)Δ(1)(ν = 0) states is discussed and a comparison of the experimentally determined properties of the X(3)Δ(1)(ν = 0) state with those predicted as a prelude to the electron electric dipole moment (eEDM) measurements [J. Lee, E. R. Meyer, R. Paudel, J. L. Bohn, and A. E. Leanhardt, J. Mod. Opt. 56, 2005 (2009)] is given.     

On the hyperfine structure in the configuration 5f 3 6d7s 2 of neutral uranium

A Laser Induced Fluorescence experiment in an atomic beam has yielded experimental values of hyperfine structure (hfs) constants A and B for 28 low odd levels and 22 even levels, with an accuracy around 10^–5 cm^–1 for A and 10^–3 cm^–1 for B. A Condon Slater Racah parametric interpretation of the hfs data concerning 22 of these levels, on the basis of the configuration 5f ³ 6d 7s ², has provided values of monoelectronic parametersa _5f ⁰¹ ,a _5f ¹² ,a _6d ⁰¹ ,b _5f ⁰² ,b _6d ⁰² ,b _6d ¹³ . A least square fit calculation has been compared to the values deduced from Dirac-Fock monoelectronic radial integrals. The fit represents 18 A (16 B) values ranging from –0.42 to –2.96 mK (from –41 to 156 mK) with discrepancies less than 0.2 mK (8 mK).     

Investigation of the structure and 13C NMR spectrum of 5-phenylbarbituric acid

Crystals of 5-phenylbarbituric acid, 2 , are monoclinic, space group C2/c. There are 8 molecules in the unit cell of dimensions a = 20.218(3), b = 7.477(1), c, = 12.443(2), β = 111.54° and V = 1749.6(4) ų. Full-matrix least-squares refinement has reached R = 0.039. This is the first barbiturate reported to exist in the enol form in the solid state. There is extensive hydrogen bonding between adjacent molecules which gives rise to very dense crystals. The ¹³C-nmr spectroscopy shows that 2 exists in both enol and keto forms in DMSO solution.     

Reaction of CF3 radicals with C2H6

The hydrogen transfer reaction between C₂H₆ and CF₃ radicals, generated by the photolysis of CF₃I, has been studied in the temperature range 298–617 K. The rate constant, based on the value of 10^13.36 cm³ mol^?1 s^?1 for the recombination of CF₃ radicals, is given by where k₂ is in cm³ mol^?1 s^?1 and E is in J mol^?1. These results are compared with those previously reported, and the following best value for k₂ is recommended: